1. Field of the Invention
The present invention relates to ink-jet inks. More particularly, the present invention relates to ink-jet inks employed in an ink-jet recording process and an ink-jet recording apparatus.
2. Description of the Related Art
Ink jet printing is a non-impact means of generating images by directing small droplets or particles in rapid succession onto the surface of a substrate. There are various possible ways of generating and projecting droplets, but only two main systems appear suited to real production applications.
The first uses a continuous stream of droplets which are electrostatically charged as they leave the nozzle and are deflected in flight by the application of a high voltage to a set of deflector plates. The charge on each individual droplet can be varied so that as the stream passes through the deflector plates, which are maintained at a constant electromagnetic potential, the trajectory of each drop (and hence its point of contact on the substrate) can be accurately controlled.
The other technique is known as "drop-on-demand" or "impulse" printing. The drops are not normally charged or deflected in their travel to the substrate. Thus, a matrix consisting of a bank of nozzles is required to create the image.
The inks used must have physical properties suitable for jet formation and streaming while being capable of producing sharp, dense, and permanent images. In addition, they should be stable on storage and must present no long-term health, or short-term chemical, hazard.
For accurate and consistent drop formation, the inks must possess a careful balance of rheology, surface tension, and (for continuous jet machines) conductivity. Flow should ideally be Newtonian, but viscosity may vary from machine to machine. Impulse jets place more critical requirements on ink viscosity than continuous jets because of their mode of operation which demands that they maintain a stable viscosity during long periods of use. Unfortunately, viscosity varies with temperature; and, for this reason, a number of impulse jet machines incorporate thermostatically controlled nozzles.
Surface tension is also a critical factor in the formation and maintenance of discrete drops. Water, at 72 dynes/cm, would provide the optimum surface tension; but, because it is blended with dyes, resins, and additives, this figure is not achieved. A more approachable figure is 50-60 dynes/cm. Solvent-based inks for continuous drop printers have even lower surface tensions.
In many applications, prints must dry quickly. This does not present too much of a problem with continuous jet printers, as fast drying solvents such as ethanol or methyl ethyl ketone (MEK) can be used. However, an ink based on such fast drying solvents would dry in the nozzles of an impulse jet printer; and, therefore, it is difficult to formulate a true "fast-drying" "drop-on-demand" ink. Most impulse jet inks are based on slow evaporating glycols to minimize evaporation.
Glycol and water based ink-jet inks dry by absorption into the paper. On more absorbent papers, drying will be quicker, but there will be a greater tendency for the ink to spread through the paper fibers giving poor ink definition and strike through. By formulating to the limits of the viscosity and surface tension specifications, it is possible to minimize this problem. Also, it can be avoided, or minimized, by using less absorbent paper.
In the case of non-absorbent surfaces, binder choice is critical. Almost all solvent based ink-jet printing inks contain resinous binders to give key and hardness. Binder resins are selected for their general adhesion and solution properties and include acrylics, polyamides, and maleics.
Most ink systems contain soluble dyes, which give satisfactory color properties for most work. Color strength is limited by the solubility of the dye, but with careful selection ranges of bright strong colors can be produced. Pigments are rarely used, because they present flow, clogging, nozzle wear, and stability problems.
Dye solubility in water tends to be dependent on pH, and ink-jet systems are normally adjusted to be neutral or slightly alkaline. To prevent pH variation due to absorption of carbon dioxide from the atmosphere, such systems may also be buffered with additives such as sodium carbonate. Also, pH can have an effect on the corrosiveness of the ink system in contact with certain metals, and this must be borne in mind when formulating for specific machines. The Printing Ink Manual (Fourth Edition, 1988) gives the following as typical ink-jet ink formulations:
TABLE I __________________________________________________________________________ Typical Ink Formulations __________________________________________________________________________ A. Solvent-based Continuous Jet Ink Aniline Blue 3.0 Solvent-soluble dye Phenol-formaldehyde polymer 6.0 Film-forming polymer to give resistance and adhesion to substrate Alcohol 49.5 Solvent Dimethylformamide 41.0 Solvent Soluble electrolytes 0.5 Conductivity aid 100.0 Final characteristics: Viscosity, 2.1 cp. at 20.degree. C. Conductivity, 1200 micromhos/cm Surface tension, 25 dynes/cm at 20.degree. C. B. Water-based Continuous Jet Ink Direct Black Dye 4.25 Distilled Water 83.15 Polyethylene Glycol 5.00 Crusting Inhibitor N-methylpyrrolidone 4.00 Dye Solvent Ethylene Glycol Monobutylether 3.00 Paper Penetrant Sequestering Agent 0.20 Heavy Metal Suppressor Buffering Agent 0.30 pH Control Biocide 0.10 Anti-mold 100.00 Final characteristics: Viscosity, 2.28 cp. at 20.degree. C. Conductivity, 11,000 micromhos/cm Surface tension, 43.5 dynes/cm pH, 10.3 C. Impulse (Drop-on-Demand) Jet Ink Direct Dyestuff 3.0 Soluble Dyestuff Polyethylene Glycol 14.0 Anti-clogging Solvent Diethylene Glycol 12.0 Humectant N-methylpyrrolidone 15.0 Dye Solubiliser Biocide 0.1 Anti-fungal Buffering Agents 0.3 pH Control Polyvinyl Alcohol 3.0 Viscosity Controller Triethanolamine 1.0 Surface Tension Controller Distilled Water 51.6 Solvent 100.0 Final characteristics: Viscosity, 9.0 cp: at 20.degree. C. Surface tension, 45 dynes/cm pH, 10.5 __________________________________________________________________________
Typical dyes used in ink-jet ink formulations are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. 4,761,180 Dyes Containing Tetramethylammonium Cation for Ink-jet Printing Inks U.S. Pat. No. 4,836,851 Dyes Containing Polyhydroxyl Groups for Ink-jet Printing Inks U.S. Pat. No. 4,994,110 Dyes Containing Lithium for Ink-jet Printing Inks U.S. Pat. No. 5,098,474 Dyes Containing Alkylamino Groups for Ink- jet Printing inks ______________________________________
Although dyes, rather than pigments, have been the traditional ink colorant of choice for ink-jet printers for fluidity and anti-clogging requirements, dyes do present several disadvantages. They may smear when brushed by an oily finger. They are water-soluble and remain so after drying. So, they redissolve upon contact with water and will run when exposed to a water spill. Also, dye images smear on contact with felt tip pen markers. Therefore, dyes may make the ink-jet ink exhibit poor color-fastness. In addition, they exhibit poor light stability relative to pigments and are known to fade even under conditions of office fluorescent lighting. Thus, there are several disadvantages with the use of dye-based ink-jet inks, many of which prohibit their use in applications requiring fast drying times and improved light-fastness, or greater light stability.
To improve the color-fastness of ink-jet prints, manufacturers are developing pigment-based inks. U.S. Pat. No. 5,172,133, for example, teaches ink-jet recording with an ink composition containing pigment and a water-soluble resin in specific weigh proportions; and U.S. Pat. No. 5,529,616 teaches a pigment-based ink-jet ink containing particular mixtures of alcohols to achieve desirable surface tension. Due to the low ink viscosity needed for this mode of printing, however, jet inks generally contain relatively low levels of resin. This makes binding of the pigment onto the paper difficult. Weak bonding of pigment to paper results in poor color-fastness, or water-fastness.
A number of methods have been proposed to immobilize the colorant of an ink jet ink on a substrate. Such known methods include metal ion chelating (U.S. Pat. No. 4,694,302), anionic/cationic reactions (U.S. Pat. No. 5,623,294), and polymeric reactions (U.S. Pat. No. 5,629,359).
Problems have been encountered with the practice of the prior art. In practicing the method of metal ion chelating, a restriction exists in that the practice is limited to environments in which the metal ion can exist in a stable form long-term. In practicing the method of anionic/cationic reactions, a restriction exists as to where the anionic or cationic component can be reside because of adverse reactions that can occur, for example, between the component and the colorant in the ink or between the component and an acidic or basic component in the substrate. In practicing the method of polymeric reactions, problems with long-term stability of components can occur, the use of heat or time may be required to achieve polymer formation, or hazardous monomers may be employed. The practice of all three methods can produce printed images that are not resistant to water and accent markers, that have poor print quality, and that have low optical densities.
Therefore, an object of the present invention is to provide recording methods and ink and liquid compositions capable of overcoming the foregoing problems and obtaining a printed image with at least: 1) complete wet (water) rub resistance, 2) complete resistance to accent markers possessing either an acidic or basic pH, 3) sufficient optical density, and 4) excellent quality. Furthermore, the present invention intends to achieve at least five goals with respect to the printing process: 1) substrate independence, 2) fast development of the desired properties of the printed image without the use of heat or excessive time, 3) alternatives as to where the components of the ink and liquid compositions can reside, 4) flexibility as to how the components can be combined, and 5) stability of the components. More particularly, it is an object of the invention disclosed herein to provide a two-component ink-jet printing ink system that provides improvements in water-fastness and wet rub resistance.